1. Field of the Invention
The present invention relates generally to the fields of cancer biology, pharmacology, and chemotherapy. More specifically, the present invention relates to delivery of chemotherapeutic retinoids to the lungs by aerosolized liposomes.
2. Description of the Related Art
Retinoids are natural and synthetic analogs of vitamin A that normally play critical roles in growth, vision, reproduction, differentiation, and immune functions (1, 2). A relationship between vitamin A and cancer was first noted when experimentally induced vitamin A deficiency was shown to lead to preneoplastic lesions and neoplasms (3, 4). Retinoids modulate the in vitro growth and differentiation of a variety of transformed cell types, including melanomas, leukemias, and squamous carcinomas (1, 2, 5-7, 9, 10). Based on this property, retinoids have been used effectively in therapeutic regimens for prevention and treatment of various dermatological conditions, acute promyelocytic leukemia, as well as squamous cell carcinomas (SCC) of the aerodigestive tract, skin and cervix (6, 7, 10).
Epithelial tissues undergo biochemical, morphological and functional changes if deprived of or treated with an excess of retinoids. Because squamous cell carcinomas are derived from epithelial cells, they may be ideal targets for treatment with retinoids. Epithelial cancers of the head and neck or lungs are a devastating group of diseases that account for approximately 30% of cancer deaths (8). Retinoids are known to be potent modulators of epithelial differentiation and carcinogenesis (9). For example, 13-cis-retinoic acid is potent in suppressing oral carcinogenesis as well as preventing second primary tumors (10). Clinical trials using retinoic acid for the treatment of oral leukoplakia, a premalignant lesion of oral cancer, showed regression of existing disease and prevention of new disease and progression of the disease (11). This effect i s reversed when the treatment is withdrawn.
However, like many anticancer drugs, administration of ATRA is associated with undesirable toxic effects both in experimental models and in patients (11, 12). Liposomes have been evaluated both clinically and experimentally as a delivery system for mitigating the toxic effects associated with administration of drugs such as doxorubicin, vincristine, amphotericin, and retinoids (13-17). Liposomal incorporation of retinoids may provide an alternative way of delivering retinoids without the resulting toxic effects. The potential advantages of liposome delivery include increased activity due to specific targeting, sequestration of the drug at the target site, protection of the drug from rapid metabolism, amplified therapeutic effect due to packaging of numerous drug molecules in each liposome, and decreased toxicity due to altered pharmacokinetics (18-21). For example, free ATRA when administered in CD-1 mice, exerted toxic effects at 25-30 mg/kg of body weight. However, in liposome-encapsulated form the animals could tolerate much higher doses of ATRA (120 mg/kg) probably due to altered drug distribution in target tissues (16).
In a test of fourteen different formulations of liposomes, those composed of dipalmitoylphosphatidylcholine (DPPC) and stearylamine (SA) seemed to be the least toxic (13). The addition of the positive-charged SA dramatically increased uptake of L-ATRA, which was also well retained over a 24 hour period by squamous carcinoma cells. The L-ATRA was also biologically active and caused the differentiation of the cells in culture as well as in a spheroid model (13). Liposomal incorporation was also seen to reduce the rate of cellular and microsomal metabolism of ATRA. The amount of metabolites secreted into the medium was decreased by 15%, and the levels of intact ATRA in the cells doubled. Thus, liposomes were able to protect the drug from the metabolic enzymes of the cells (20). Microsomes isolated from the livers of free ATRA-treated rats catabolize [3H]ATRA faster than microsomes isolated from L-ATRA-treated rats (18). Similarly, in leukemia patients, the plasma drug levels are maintained over long periods of time following administration of L-ATRA (19).
Retinoic acid is currently being administered to patients as an oral formulation. This would probably allow only very low levels of the drug to reach the tumor sites in the aerodigestive tract and lungs. Oral administration of retinoids allows only low levels of drugs to reach the aerodigestive tract especially the lungs. L-ATRA can be administered intravenously, but due to its particulate nature, a major fraction of the dose is taken up by the reticuloendothelial system. If the drug could be targeted directly to the aerodigestive tract, much higher concentrations of the drug could be achieved with minimal toxicity. The advantage of the aerosol mode of delivery is that the drug is deposited more uniformly over the respiratory tract, leading to local levels of the drug that may far exceed the levels achieved by systemic administration (22). However, free ATRA due to its lipophilic properties cannot be aerosolized.
The prior art is deficient in the lack of a means to deliver ATRA directly to the lungs. The present invention fulfills this longstanding need and desire in the art.
In one embodiment of the current invention, a method is described for the delivery of chemotherapeutic retinoids to the aerodigestive tract. The retinoid is incorporated into liposomes which are resuspended as an aqueous suspension. The resulting suspension is then aerosolized and inhaled by the animal or individual in need of such treatment.
In a preferred embodiment, the retinoid is all-trans-retinoic acid (ATRA), 13-cis retinoic acid, 9-cis retinoic acid, or 4-HPR. The retinoid is incorporated into the liposomes at a concentration of 0.1 to 10 mg/ml. After formation of the aerosol, the dose of the retinoid ranges from 1 to 100 xcexcg retinoid per liter of aerosol. It is expected that this treatment shall result in a lung accumulation of 0.05 to 20 xcexcg retinoid/gram of lung tissue.
In one embodiment of the present invention, the liposomes are composed of dipalmitoylphosphatidylcholine (DPPC) and stearylamine (SA). A dipalmitoylphosphatidylcholine to stearylamine ratio of 9:1 w/w is an appropriate formulation.
Likewise, in the preferred embodiment, the retinoid is incorporated into said liposomes at a drug:lipid ratio of 1:10 (w/w).
One method of forming the aerosol is with a jet type nebulizer. An appropriate aerosol can be formed by operating said nebulizer at a flow rate of 10 liters/minute for 30 minutes Aerosolization is carried out to obtain aerosol particles with mass median aerodynamic diameters of 1 to 3 microns. The liposomes within the aerosol have diameters falling within the 100 to 1000 n m range. The aerosol can then be inhaled for a period of time ranging from 5 minutes to 2 hours.
The treatment can be applied to any tumors of the aerodigestive tract but is especially useful for the treatment of tumors of the lungs. A series of treatments will be administered to maintain at a retinoid level in the lungs of 0.05 to 20 xcexcg retinoid/gram of lung tissue. The frequency of the treatments may be multiple times per week. It is also possible to administer a systemic dose of retinoid by absorption through the lungs.